The present invention relates to the safety-relevant or fail-safe determination of a movement variable on an automatically operating installation (or machine), such as a manufacturing installation and/or conveyor installation. More particularly, the invention relates to the fail-safe determination of a movement velocity and/or the movement travel of the beam of a press brake or a similar installation having a tool which moves like a stamp. The invention also relates to a safety device and to a method for fail-safely stopping of a moving part of such an installation in response to the movement variable.
DE 100 27 156 A1 discloses a press brake or a similar machine, in which a first machine part carries out a stamp-like working movement with respect to a second machine part. This working movement can be used to shape a workpiece under pressure, or else for cutting or stamping. As will readily be appreciated, such a stamp-like working movement is highly dangerous to the operator who, for example, has to insert and align the workpiece under the moving part. In fact, accidents occur again and again, in which an operator is subjected to severe crushing, even with body parts being cut off, as a result of carelessness and/or incorrect actions. In order to prevent such accidents, the installation from DE 100 27 156 A1 has a non-contact protective device. The protective device comprises a light barrier arrangement, whose light beams run parallel to the front edge of the moving tool, with the light barrier arrangement being moved with the working movement of the tool. If one or more of the light beams of the light barrier arrangement is interrupted, the working movement is typically stopped immediately. However, the light barrier arrangement has to be deactivated (what is called muting) shortly before the press closes, because the interruption of the light beams by the workpiece would otherwise prevent the press from closing.
In order to allow older presses to be retrofitted with such a light barrier arrangement, DE 100 27 156 A1 proposes that the light barrier arrangement be deactivated (muted) when the movement velocity of the stamp-like tool is below a predetermined velocity. This is because, in the case of a press, the stamp-like tool is normally driven from its initial rest position (what is called upper dead point) at high speed towards the workpiece. However, the workpiece is shaped at a low velocity (creeping speed). According to DE 100 27 156 A1, deactivation of the light barrier arrangement solely as a function of the movement velocity of the tool has the advantage that even old presses can be easily retrofitted with the described light barrier arrangement. On the other hand, the proposed procedure is dependent on the capability to fail-safely determine the respective velocity of the tool, i.e. the movement velocity of the tool must be determined in a safety-related manner such that a dangerous state for the operator is prevented to occur even in the event of a fault or functional failure in the safety device.
In the case of the press from DE 100 27 156 A1, the velocity of the tool is therefore determined by two measurement modules, with one measurement module being in the form of an incremental shaft encoder, in which a cable which is fixed to the tool is wound up and unwound by the working movement. The rotary movement which is produced by the winding up and unwinding is detected by an incremental shaft encoder or a rotation sensor. The second measurement module has an inductive sensor, through which a magnetic measuring tape is moved, with the magnetic measuring tape likewise following the movement of the tool. Alternatively, measurement devices are disclosed which include a toothed rod, a linear potentiometer, a translucent scale with a light barrier, or an inductive sensor with a perforated strip of sheet metal.
These sensors all have in common that their physical dimensions depend on the size of the installation and the movement travel of the moving installation part. If, for example, the intention is to monitor a movement travel of 1000 mm, a correspondingly long cable, measuring tape or scale is required. The proposed measurement means are therefore physically large, and they involve complex installation with correspondingly high costs. This applies even more to the safety-relevant determination of movement variables of a moving installation part, since redundant measurement means generally have to be used, for safety reasons.
A similar installation, again in the form of a press, is known from WO 97/25568. An optical encoder disk is used here in order to monitor the working movement of the tool. The encoder disk is rotated by a chain, a cable or the like, which is attached to one end to the moving tool. This has the same disadvantages in terms of material costs, dimensions and installation effort as the installation from DE 100 27 156 A1.